1. Field of the Invention
The present invention relates to new crystalline zeolite SSZ-57 and processes employing SSZ-57 as a catalyst.
2. State of the Art
Because of their unique sieving characteristics, as well as their catalytic properties, crystalline molecular sieves and zeolites are especially useful in applications such as hydrocarbon conversion, gas drying and separation. Although many different crystalline molecular sieves have been disclosed, there is a continuing need for new zeolites with desirable properties for gas separation and drying, hydrocarbon and chemical conversions, and other applications. New zeolites may contain novel internal pore architectures, providing enhanced selectivities in these processes.
Crystalline aluminosilicates are usually prepared from aqueous reaction mixtures containing alkali or alkaline earth metal oxides, silica, and alumina. Crystalline borosilicates are usually prepared under similar reaction conditions except that boron is used in place of aluminum. By varying the synthesis conditions and the composition of the reaction mixture, different zeolites can often be formed.
The present invention is directed to a family of crystalline molecular sieves with unique properties, referred to herein as xe2x80x9czeolite SSZ-57xe2x80x9d or simply xe2x80x9cSSZ-57xe2x80x9d. Preferably, SSZ-57 is obtained in its borosilicate form, but the present novel zeolite can be synthesized in the borosilicate, aluminosilicate or all-silicate phase. The term xe2x80x9cborosilicatexe2x80x9d refers to a zeolite containing oxides of both boron and silicon. The term xe2x80x9caluminosilicate refers to a zeolite containing oxides of both aluminum and silicon.
In accordance with this invention, there is provided an improved process for the reduction of oxides of nitrogen contained in a gas stream in the presence of oxygen wherein said process comprises contacting the gas stream with a zeolite, the improvement comprising using as the zeolite a zeolite having a mole ratio greater than about 20 of an oxide selected from silicon oxide, germanium oxide and mixtures thereof and boron oxide, aluminum oxide, gallium oxide, indium oxide, titanium oxide, iron oxide, vanadium oxide or mixtures thereof and having, after calcination, the X-ray diffraction lines of Table 3. The zeolite may contain a metal or metal ions (such as cobalt, copper or mixtures thereof) capable of catalyzing the reduction of the oxides of nitrogen, and may be conducted in the presence of a stoichiometric excess of oxygen. In a preferred embodiment, the gas stream is the exhaust stream of an internal combustion engine.